Using the embryonic chick hindlimb/spinal cord system this study will define the time period and manner in which the peripheral connections of motoneurons are specified. The time at which specification occurs will be determined through surgical manipulation of the cord of motoneuron precursors and an autoradiographic analysis of cell birthdates and settling patterns with the cord motor column will show whether motoneurons are specified by their time of origin or by early position within the neural tube. Manipulations of premuscle tissue within the limb will be used to determine whether motoneurons demonstrate a specific affintiy for their correct target muscle during outgrowth. In all of these experiments, peripheral connectivity patterns will be assessed using electrophysiological and anatomical labeling techniques. A study of the development of motoneuron projection patterns in the mouse hindlimb will elucidate the role of cellular interactions in refining connectivity patterns. The timing of axon outgrowth and extent of ramification within specific muscles will be identified electrophysiologically and anatomically and correlated with specific spatial and temporal patterns of motoneuron cell death and muscle differentiation.